Phase Selective Membrane Based Separators for Portable Direct Methanol Fuel Cell Systems

Direct methanol fuel cell (DMFC) systems convert methanol into electricity at high thermodynamic efficiencies. They provide high energy density and the possibility for an instant refill of methanol. To enable an autonomous operation of a DMFC system, the gas has to be separated from the gas/liquid stream at both, anode and cathode exit. In the same way, the methanol solution of the anode loop has to be recycled and the product water of the cathode has to be recovered to compensate for the water losses in the anodic loop [1]. In the macroscopic domain, passive separation systems often use gravity to separate different phases of fluids. Such separation systems provide excellent separation properties as long as they are not subjected to mobile or portable requirements. In the domain of small energy systems, such as portable DMFC systems for off-grid power supply, orientation dependent separation units cannot be applied. New perspectives and approaches are provided by the domain of microfluidic systems because the surface forces occurring in such systems dominate over gravity forces. Capillary pressure may be used to hold back or extract a certain phase.

This contribution gives an insight into the removal of gases or liquids from gas/liquid streams by the combination of milli/micro structured channels and micro structured membranes for DMFC systems (figure 1). To achieve a complete separation of gases from a gas/liquid stream, a combination of hydrophilic walls of the supporting channel and a hydrophobic surface of the membrane is used. For the removal of liquid from a gas/liquid stream, a hydrophilic membrane and a hydrophobic main channel is applied. The aim of this work is to determine the limits of multi-phase separation by gas permeation and by liquid permeation using micro structured membranes and supporting channels. For systems that are open to the environment, the influence of environmental parameters like temperature and relative humidity is studied in experiment and model. The effect of system parameters like pressure, temperature, volume flow rates and orientation of the device on the separation and the recycling efficiency is investigated experimentally for both kinds of separation units. It is shown experimentally that, compared to other parameters, the orientation of the separation unit has only a minor effect on the separation and the recycling efficiency. Model based analysis gives a further insight into geometry based restrictions for the separation and recycling efficiency and clarifies the observed experimental results and effects.

Figure  SEQ Figure \* ARABIC 1: Position and design of phase selective separators in a portable DMFC system

References:

[1]   Zenith, F., Krewer, U., 2010. Modelling, dynamics and control of a portable

DMFC system. Journal of Process Control 20 (5), 630–642.